1. Field of the Invention
The present invention relates to a method for fabricating a display device.
2. Description of Related Art
A full color organic EL display device includes an anode electrode, a hole injection layer, a hole transport layer, an organic EL layer having R, G and B color patterns, an electron transport layer, an electron injection layer, and a cathode electrode, which are sequentially stacked on an insulating substrate.
Of these, the organic EL layer is formed by using a vacuum deposition technique or a light etching technique using a shadow mask.
However, the vacuum deposition technique has a disadvantage in that there is a limitation on a minimum value of a physical gap and a large-sized organic EL display device, and it cannot be applied to an organic EL display device having fine patterns of tens of micrometers (μm) due to, for example, a mask transformation.
The light etching technique also has a disadvantage in that the organic EL layer can deteriorate due to a developing solution and etchant even though it can form fine patterns.
In order to overcome the above problems, a method of forming the organic EL layer using a thermal transfer technique is introduced. The thermal transfer technique is one which transfers a color pattern of the transfer film onto a substrate using a heat energy generated by light emitted from a light source.
Such a thermal transfer technique includes two techniques. One is related to controlling the light source, and the other is related to a configuration of the transfer film.
A laser beam is mainly used as a light source. A pigment colorant of the transfer film is scanned by the laser beam according to a desired pattern and transferred to the substrate, thereby forming a color pattern on the substrate.
U.S. Pat. No. 5,521,035 discloses methods of preparing color filter elements using laser induced transfer of colorants, wherein a Nd:YAG laser is used as a light source. The Nd:YAG laser forms a gaussian shaped beam having a Gauss distribution. The gaussian shaped beam, for example, having a diameter of more than 60 micrometers (μm), shows a characteristic that an energy distribution is gentler as it becomes more distant from a central point thereof. When the color pattern is formed using the gaussian shaped beam having a predetermined diameter, an intensity of the laser beam at an edge of the color pattern becomes weak. Consequently, the edge of the color pattern transferred is not clear and has a bad quality.
Techniques for a configuration of the transfer film are disclosed in U.S. Pat. No. 5,220,348 of D'Aurelio et al., U.S. Pat. No. 5,256,506 of Ellis et al., U.S. Pat. No. 5,278,023 of Bills et al., U.S. Pat. No. 5,308,737 of Bills et al., U.S. Pat. No. 5,998,085 of Isberg et al., U.S. Pat. No. 6,228,555 of Hoffend et al., U.S. Pat. Nos. 6,194,119 and 6,140,009 of Wolk et al., U.S. Pat. No. 6,057,067 of Isberg et al., U.S. Pat. No. 6,284,425 of Staral et al., U.S. Pat. Nos. 6,270,934, 6,190,826, and 5,981,136 of Jeffrey et al.
The techniques for a configuration of the transfer film are focused on a thermal transfer donor element which includes a base layer, a radiation absorber, a transfer layer and a gas-generating polymer layer. So, the techniques for a configuration of the transfer film do not suggest an improvement on reducing a deterioration of the edge portion of the color pattern.
Meanwhile, a conventional full color organic EL display device is manufactured such that a transparent electrode made of, for example, indium tin oxide (ITO), is formed over a thin film transistor (TFT) array substrate, and an insulating layer is formed over the whole surface of the substrate to expose a portion of the transparent electrode, and finally an organic EL layer is formed on the exposed portion of the transparent electrode.
An edge portion of the transparent electrode is covered with the insulating layer. This prevents a deterioration of the organic EL layer to increase a life span of the low molecular organic EL display device, and forms a wall to prevent a leakage of a solution during an ink-jet printing process to form the organic EL layer in the high molecular organic EL device. The technique is disclosed in EP 969701, SID 99 Digest P. 396, IEEE '99 P. 107, and other similar documents.
Meanwhile, methods of manufacturing the full color organic EL display device using a laser transfer (i.e., thermal transfer) technique are disclosed in Korean Patent no. 10-0195175, Korean Patent Application no. 2000-49287, and U.S. Pat. No. 5,998,085. The transfer film is in contact with the TFT array substrate, and is scanned using a laser beam. The laser beam is absorbed into a light absorber of the transfer film and so is converted into a heat energy. An organic electroluminescent material is transferred from the transfer film to the substrate by the heat energy to thereby form a color pattern of the organic EL layer.
In the conventional art, a thickness of the insulating layer is set from 500 nm to 1000 nm or more than 1000 nm in consideration of a parastics capacitance. Due to the thick thickness of the insulating layer, defects in the edge of the organic thin layer occur in the case that the organic EL layer is formed using the laser transfer technique.
These defects can result from a characteristic of an underlying layer formed under the organic EL layer. For example, the defects occur when the underlying layer is formed non-uniformly, when the organic EL layer is not formed on the edge portion of the insulating layer to form a hole, or when the underlying layer is separated from other layers.
U.S. Pat. No. 5,684,365 discloses a method of preventing defects of the organic EL layer which can occur in a boundary between the transparent electrode and the insulating layer.
FIG. 1 is a cross-sectional view illustrating an organic EL display device shown in U.S. Pat. No. 5,684,365. Referring to FIG. 1, a semiconductor layer 120 is formed on an insulating substrate 100 in the form of an island. The semiconductor layer 120 includes source and drain regions 124 and 125, respectively, and is made of poly silicon. A gate insulating layer 130 is formed over the whole surface of the insulating substrate 100 and covers the semiconductor layer 120. A gate electrode 135 is formed on the gate insulating layer 130. An interlayer insulating layer 140 is formed on the gate insulating layer 130 and covers the gate electrode 135. Contact holes 144 and 145 are formed to expose a portion of the source region 124 and a portion of the drain region 125, respectively. A source electrode 154 is electrically connected to the source region 124 via the contact hole 144. A pixel electrode 170 is electrically connected to the drain region 125 via the contact hole 145. A passivation layer 180 is formed over the whole surface of the insulating substrate 100 to expose a portion of the pixel electrode 170, thereby forming an opening portion 185. An organic EL layer 190 is formed on the exposed portion of the pixel electrode 170 through the opening portion 185. A cathode electrode 195 is formed to cover the organic EL layer 190.
An edge of the passivation layer 180 defining the opening portion 185 has a taper angle of 10° to 30°. The tapered edge of the passivation layer 180 serves to improve an adhesion of the organic EL layer 190, thereby preventing defects of the organic EL layer 190.
However, in the case of forming the organic EL layer using the laser transfer technique, there are still problems in that the defects in the edge of the organic thin layer occur when the thickness of the insulating layer is more than 500 nm, even though the insulating layer is formed in order to form a taper angle in the edge of the passivation layer.